I had the same problem, however using the solution immudium pointed out above, didn't even allow me to manually manage the files on my Nexus 7. Using sys-fs/mtpfs-1.1, and manually mounting the device appeared to mount the internal storage and show the correct directory structure, but it showed no files, (and was read only of course). So it definitely wasn't working right. (BTW, same when I tried accessing the Nexus 7 with gMTP)

My solution was to use jmtpfs instead. I used the ebuild from this Gentoo forum post, copied to my local overlay, to install sys-fs/jmtpfs-0.4 into my system, and then modified my /etc/fstab line to;

Code:

jmtpfs /mnt/nexus fuse user,noauto,allow_other 0 0

So now, after I plug my Nexus 7 in, when I issue the command;

Code:

mount /mnt/nexus

I can manage the files on my device. BTW, I don't know if it matters, but in order to keep my mtp support up to date, I keyworded and emerged media-libs/libmtp-9999, and then I made sure to create a symbolic link in my udev directory for the rules file that is installed with it;

to mount my galaxy nexus. It's not an automount solution but it works and is the best solution I have for now. An alternative I suppose is to root the phone and then copy files over wifi via the sshdroid app but I haven't tried this.